Crustal Thickening in an Active Margin Setting (Philippines): the Whys and the Hows

Home , Accretion (geology), Luzon Volcanic Arc

260 by C.B. Dimalanta and G.P. Yumul, Jr. Crustal thickening in an active margin setting (Philippines): The whys and the hows

National Institute of Geological Sciences, College of Science, University of the Philippines, Diliman, Quezon City, Philippines 1101. E-mail: [email protected]

A synthesis of crustal thickness estimates was made parts of the Philippines could be attributed to subduction processes? recently utilizing available field, geochemical, seismic- What is the effect of collision events on crustal growth? What other processes contribute to the thickened crust? This paper forwards an ity, shear wave velocity and gravity data in the Philip- explanation on the different mechanisms and processes, which can pines. The results show that a significant portion of the account for the thickness of crust in the Philippines. This, hopefully, Philippine archipelago is generally characterized by can help in our understanding of how crustal growth processes occur in an active margin setting. crust with a thickness of around 25 to 30 kilometers. However, two zones, which are made up of a thicker crust (from 30 to 65 km) have also been delineated. The Tectonic setting of the Philippine Luzon Central Cordillera region is characterized by thick crust. Another belt of thickened crust is observed archipelago in the Bicol-Negros-Panay-Central Mindanao region. The convergence of the Sundaland-Eurasian margin with the Philip- This paper examines the interplay of tectonic and mag- pine Sea Plate resulted in the different features that comprise the matic processes and their role in modifying Philippine Philippine archipelago. These include magmatic arcs, subduction arc crust. The processes, which could account for the zones, collision zones and marginal basins. Based on seismicity and volcanism, the Philippine archipelago is divided into the seismically observed crustal thicknesses, are presented. The contri- active Philippine Mobile Belt and the aseismic Palawan microconti- butions of magmatic arcs as compared to the contribu- nental block. The Philippines is bounded on the east by the west- tion of the emplacement and accretion of ophiolite com- ward-dipping East Luzon Trough—Philippine Trench along which plexes to crustal thickness are also discussed. the Philippine Sea Plate is obliquely subducting. NUVEL-1 measurements show that the Philippine Sea Plate, in the region northeast of Luzon, is moving northwest at a rate of approximately 7 cm per year. But southeast of Mindanao, the plate motion increases to ~ 9 cm Introduction per year (Bautista et al., 2001). Several marginal basins are found on the western portion of the archipelago. These include the South The evolution of continental crust has been a problem of long stand- China Sea, Sulu Sea and Celebes Sea basins, which are subducting ing interest to various geoscientists. In studies, which try to address along the east-dipping Manila-Negros-Sulu-Cotabato trenches (Fig- the question of crustal growth, the addition of materials to the crust ure 1). The different ophiolite complexes, which can be found in var- is attributed to several mechanisms. Juvenile crustal material is pro- ious parts of the country, are believed to have originated from the duced in island arcs and oceanic plateaus (Rudnick, 1995; Condie, marginal basins or their ancient counterparts surrounding the archi- 1998). These features eventually collide with and become sutured to pelago. The magmatic arcs associated with the subduction zones as continents thereby leading to the growth of continental crust. Contri- well as the different ophiolite complexes are discussed in the suc- butions from magmatic underplating and overplating (Wilson, 1994; ceeding sections. Excess stress resulting from the subduction of Rapp and Watson, 1995) as well as from the emplacement of ophio- oceanic crusts along these subduction zones is absorbed by the lites have also been recognized (Dewey and Windley, 1981). Pro- Philippine Fault Zone and other related fault structures (Fitch, 1972). cesses such as arc magmatism, oceanic plateau accretion, intraplate volcanism, crustal underplating, accretion of ophiolites, sediment Magmatic arcs subduction, tectonic erosion and delamination may operate singly or in combination to contribute to crustal growth. In the Philippines, The Philippine island arc is built on oceanic crust, which had relatively few studies have looked closely into the problem of crustal been modified by several episodes of magmatism (Figure 2a). Arc growth. Crustal thickness estimates are limited to specific areas and magmatic activity that characterized the evolution of the Philippines are mostly derived from geochemical, seismicity, focal mechanism served to thicken the crust of this island arc system. and gravity data (Bautista et al., 2001; Barretto et al., 2000). A recent The earliest volcanic activity recorded in arc volcanic rocks compilation of geochemical, seismicity and gravity data to estimate from the Philippine Mobile Belt commenced during the Cretaceous crustal thickness in various parts of the Philippines show that a sig- time (Wolfe, 1981; Deschamps and Lallemand, 2002). A record of nificant portion of the Philippine archipelago is characterized by this volcanic activity is preserved in arc rocks from Cebu, which crust with thicknesses ranging from 15 to 30 km. However, areas yielded a late Early Cretaceous age (~108±1 Ma) (Walther et al., underlain by a thicker crust (from 30 to 65 km) have also been rec- 1981). Evidence for this activity is also preserved in Catanduanes ognized from a recent compilation (Dimalanta & Yumul, 2003). Island where an andesite sample gave a K-Ar age of 121.09±2.61 Ma The complex geodynamic and tectonic setting of the Philip- (David, 1994). There is also a record of magmatism preserved in the pines necessitates an understanding of the role of tectonic and mag- Luzon Central Cordilleran region during the Late Cretaceous period matic processes to crustal growth. The following questions need to (K-Ar dating of schist sample: 82.6±20.6 m.y.) (Wolfe, 1981). Rin- be addressed. How much of the observed crustal thickening in some genbach (1992) has also reported an Upper Cretaceous age (Cam-

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panian-Maastrichtian) for the pelagic foraminifera found in volcani- clastic sediments in Angat, Southern Sierra Madre (Figure 1). The subduction of the Philippine Sea Plate along the proto-East Luzon Trough is believed to be responsible for this magmatic episode. This is consistent with the existence of a well-developed accretionary prism, which cannot be attributed to the rejuvenated, present-day East Luzon Trough (Balce et al., 1976; Yumul et al., 2003c). A Late Cretaceous volcanic arc sequence was mapped in the Caramoan region in southeastern Luzon. 40Ar-39Ar dating of amphi- bole separates from a basaltic lava flow sample yielded an age of 91.1±0.5 Ma (David et al., 1997). Cretaceous magmatism is also recorded in Rapu-rapu where diorite intruding the ultramafic rocks yielded a 77.1±4.6 Ma age (David et al., 1996). Further south of that, three magmatic episodes have been recognized in the Southeastern Luzon volcanic arc. The oldest magmatism is recorded at 68.6 Ma from Balatan, which is in the central portion of the Southeastern Luzon volcanic arc. This age was based on the result of a K-Ar dating on a dioritic sample (Japan International Cooperation Agency—Metal Mining Agency of Japan, 1999). Andal (2002) sug- gested that this magmatic episode might be attributed to the subduction of the northern margin of the Indo-Australian Plate below the Philippine Sea Plate. In Central Philippines, the oldest dated rocks are found in Guimaras Island with an age of 59±2 Ma (Wolfe, 1981) (Figure 1). The next most significant period of magmatism is represented by the Paleogene (Paleocene-Oligocene) and Neogene magmatic belts (Figure 2a) (Wolfe, 1981; Bureau of Mines and Geosciences, Figure 1 Map of the Philippines showing its general tectonic 1982; Yumul et al., 2003b). Most of the Paleogene magmatic belts features. The Philippines is divided into the seismically active are found in eastern Philippines (e.g. Sierra Madre, Bicol, Samar- Philippine Mobile Belt (horizontally lined pattern) and the Leyte and eastern Mindanao). Volcanism along eastern Philippines, aseismic Palawan microcontinental block (dark gray shaded from Bicol to Leyte, is mostly related to the westward subduction of area). The rate and direction of convergence as determined from the Philippine Sea Plate along the Philippine Trench (Divis, 1980). the GPS measurements (adopted from Bautista et al., 2001) is Oligo-Miocene magmatism, which are significant because of the indicated by the vectors. Areas from which Cretaceous magmatic gold deposits in the Philippines that are related to these intrusions, ages were determined (using K-Ar or Ar-Ar dating = filled circle; are represented by rocks in the Luzon Central Cordillera, eastern using paleontologic dating = filled square) are also shown on the Negros—western Panay, Eastern Mindanao and Cotabato (Figure map (Walther et al., 1981; Wolfe, 1981; David, 1994; David et al., 2a) (Mitchell and Leach, 1991; Yumul et al., 2003a). 1996; 1997).

Figure 2 A—Map showing the ancient and modern magmatic arcs (modiﬁed from Aurelio, 2000; Yumul et al., 2003b). B—Map showing the different ophiolite/ophiolitic complexes that comprise the Philippine archipelago (Yumul, 2003). C—Map summarizing the thickness of the crust in various parts of the Philippines. Area enclosed in dashed lines corresponds to crustal thickness of ~15 to 29 kilometers. Gray shaded areas indicate crustal thickness from 30 to 65 kilometers (modiﬁed from Dimalanta and Yumul, 2003). See text for details.

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The Neogene magmatic arc from Tablas–Western Panay, which was interpreted by Mitchell and Leach (1991) to be a contin- Crustal growth by magmatic and uation of the Miocene Luzon arc, is deemed to be associated with the subduction of the South China Sea plate along the Manila-Negros tectonic processes Trench. In Cotabato, the Neogene magmatic belt is attributed to subduction of the Celebes Sea plate along the Cotabato Trench. Sajona Available field, geophysical (e.g. seismicity, gravity, seismic refrac- (1995) proposed that volcanoes in central Mindanao, which cannot tion) and geochemical data (e.g. CaO6.0 and Na2O6.0) were utilized be attributed to any active subduction zone, may be explained as the to estimate crustal thickness in various parts of the Philippine archi- products of partial melting of a detached slab beneath Mindanao. pelago. The resulting values were subsequently used to obtain arc Holocene magmatic belts are noted north of Luzon in the magmatic addition as well as ophiolite accretion rates. This kind of Batanes islands, Zambales-Bataan-Mindoro and Negros-Zam- work had not been done previously for the entire Philippines. The boanga-Western Mindanao (Figure 2a). The Holocene volcanism in results reveal that certain portions of the Philippines are underlain by Luzon is attributed to the subduction of the South China Sea oceanic thickened crust (>30 km) (Dimalanta and Yumul, 2003). From the lithosphere along the east-dipping Manila Trench (Yumul et al., observed crustal thicknesses, this paper presents and examines the 2003b). The volcanoes in eastern Negros have been proposed to be various mechanisms responsible for the thickening of the crust in the related to the subduction of the Sulu Sea basin along the Negros archipelago. Trench (Mitchell and Leach, 1991). The Zamboanga-Western Min- Figure 2c shows that a zone of thickened crust characterizes the danao arc is explained by subduction along the Sulu Trench (Aure- Luzon Central Cordillera region. The oldest magmatic activity in lio, 2000). this region began during the Late Cretaceous period. Substantial These different episodes of magmatic activities commencing from the Cretaceous to Recent have produced a considerable volume thickening of the crust was brought about by subsequent magmatic of materials (1.85 x 106±256,000 km3). This translates to arc mag- episodes from the Paleocene to Upper Miocene, which are recorded matic addition rates ranging from 30 to 95 km3/km/m.y. (Dimalanta among the various sedimentary and igneous rocks in this region and Yumul, 2003). (Dimalanta, 1996). Such multiple phases of magmatism concen- trated along linear belts corresponding to magmatic arcs can explain Ophiolites and ophiolitic complexes Another mechanism that results into crustal addition, aside from arc magmatism, is the emplacement of ophiolites (Figure 3a). The presence of ophiolite and ophiolitic complexes has been reported by workers in various parts of the Philippine archipelago (e.g. Balce et al., 1976; Yumul et al., 1997; Tamayo, 2001; Dimalanta and Yumul, 2003). These exposed oceanic lithospheres make up a large part of the basement lithologies in these areas. Ophiolite complexes are believed to have been emplaced and accreted through onramping, upwedging, compression and faulting (Figure 3a) (e.g. Moores, 1982; Cannat, 1993; Hacker et al., 1996). Balce et al. (1976) presented a zonation of ophiolite complexes based on the geographic distribution of these ophiolite units. Recent data, however, has led to the grouping of the ophiolites, on the basis of their ages as well as other characteristic features (e.g. presence of associated metamorphic sole; related to mélanges; manner of emplacement mechanism). The ophiolites are grouped into the following belts: Eastern Philippines, Central Philippines, Zambales, Panay-Mindoro, Palawan-Zamboanga (Figure 2b). This latest zonation suggests a progressive younging of the ophiolites in a westward direction (Yumul, 2003). Despite the presence of numerous ophiolites and ophiolitic complexes in different parts of the archipelago, the volume of material resulting from the emplacement of these ophiolites (724,000 ± 43850 km3) is considerably less than the amount produced during arc magmatism (1.85 x 106±256,000 km3) (Dimalanta and Yumul, 2003). This is understandable considering that not all of the materials become accreted or emplaced. Some crustal subtraction takes place when a certain portion of the materials becomes destroyed or lost as a result of subduction, tectonic erosion or subduction knead- ing (e.g. Charvet and Ogawa, 1994; Clift et al., 2003). The simplified computation of the volume of material related to ophiolite and ophiolitic complexes yielded ophiolite accretion rates ranging from 2 to 19 km3/km/m.y. (Dimalanta and Yumul, 2003). Figure 3 Schematic diagram showing crustal addition by different These rates are lower compared to the few rates that are available mechanisms. A—Emplacement of ophiolite by strike-slip faulting elsewhere. Godfrey and Klemperer (1998) derived an ophiolite accre- (e.g. Zambales ophiolite) leads to the addition of materials. B—The tion rate of 50 km3/km/m.y. for the Great Valley ophiolite in Califor- significantly thickened crust in Bicol, southeastern Luzon may be nia. Although arc magmatic processes dominate crustal growth in the accounted for by numerous episodes of arc magmatism in the region. Philippines, present available data demonstrate the critical role C—The collision between the Palawan microcontinental block and played by amagmatic crustal growth processes, that is, the accretion the Philippine Mobile Belt did not result into crustal thickening. of ophiolites. This is made more significant considering the fact that Further work needs to be done in order to determine whether the the Philippine island arc system is made up of several collision and absence of crustal thickening is due to crustal subtraction such as suture zones. These areas are, almost always, characterized by the subduction erosion or other related process. presence of ophiolites and, to a certain extent, mélanges.

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why there is a thickening of the crust in this part of the Philippines. addition of materials by arc magmatism is more substantial com- This mechanism may also account for the thickened crust that can be pared to those added during the accretion and emplacement of ophi- observed in other parts of the Philippines like the central Mindanao olites. region. The boundary between the thick and thin crustal regions coin- The significantly thickened crust in Bicol, southeastern Luzon cides with recognized sutures (e.g. Siayan-Sindangan Suture Zone in may be accounted for by the numerous episodes of arc magmatism, western Mindanao) or major faults (e.g. the Philippine Fault Zone) in both ancient and recent, which the region had experienced (Figure some places (Figure 4). In areas where a physical boundary between 3b). This area is also characterized by the presence of several ophio- the thin and thick crustal regions is not observed, this might be attrib- lite complexes. The emplacement of these ophiolite complexes has uted to the complex history involving magmatic, structural and sed- further served to increase the thickness of the crust in this part of the imentation processes which makes the boundary not readily observ- archipelago. able. This is a subject that can be looked into in future research work. It is interesting to note that in the zonation of the Philippines The collision between the Palawan microcontinental block and based on crustal thickness, central Philippines (i.e. Negros, Panay, the Philippine Mobile Belt during the Early Miocene is one of the Cebu and Bohol) is shown to be made up of thick crust (30–65 km) more significant events in the evolution of the Philippines (Bellon (Figure 2c). One of the magmatic belts passes through this region. and Yumul, 2000). Hence, it may be worthwhile to consider whether The islands are also characterized by the presence of ophiolite suites. the collision event had any effect on the thickening of the crust. In In eastern Central Philippines, Leyte is characterized by Neogene the crustal thickness zonation map, the Mindoro-Romblon region, volcanism but no ancient magmatic activity has been reported for the which is supposedly the site of collision (Yumul et al., 2003c), does island. Hence, the crust here is only less than 30 kilometers thick. not show any significant thickening of the crust. The material being Although Leyte is also floored by an ophiolite basement, this has not "crumpled" as a result of the collision has not sufficiently thickened caused a substantial increase in the thickness of the crust. or stacked up to cause a significant increase in the thickness of the It would, thus, appear that crustal thickening in the Philippines crust in the area surrounding the collision zone. Current studies are is the result of the various episodes of arc magmatism occurring in being undertaken to investigate what processes are responsible for almost the same region or belt, which modified the crust. In addition, this result. However, some initial models to explain the absence of the emplacement of ophiolites through various mechanisms has put crustal thickening from collision are proposed here. This could be in a considerable amount of material to cause crustal thickening. due to the fact that although the collision is still in progress, the However, based on present available rates, it is apparent that the resulting crustal thickening has not been readily registered. Another possibility is that the plates that are colliding are initially thin, thus, although considerable thickening of the collided portions occurred, with respect to the surrounding plates, they do not manifest anom- alous thickness. Although accretion can result from collision, mechanisms such as delamination, subduction erosion and other related processes might also explain the observed result (Figure 3c). This will have to be addressed by future work.

Conclusions

The most recent estimates of arc magmatic addition rates in the Philippines reveal values from 25 to 60 km3/km/m.y. Ophiolite accretion rates have been estimated to range from 2 to 19 km3/km/m.y., which are considerably higher compared to rates obtained elsewhere. From the volumes of crustal material produced by arc magmatism, it would seem that crustal growth in the Philip- pines has been dominated by arc magmatism. However, contributions from the emplacement and accretion of ophiolites are quite sig- niﬁcant. The combination of these processes might also explain the occurrence of thickened crust in certain portions of the archipelago (i.e. Luzon Central Cordillera and the Bicol-Panay-Masbate areas). The "crumpling" effect caused by the collision event between the Palawan microcontinental block and the Philippine Mobile Belt has not resulted in thickening of the collided crusts. The manner in which island arcs evolved, as presented here, through magmatic and amagmatic processes may help us understand their ultimate fate of being accreted to the continents.

Acknowledgements

The University of the Philippines–National Institute of Geological Sciences research grants to CBD and GPY are acknowledged. Most Figure 4 Schematic diagram showing the boundary between the of the work on ophiolites and arc volcanoes were funded by the thin and thick (horizontally lined area) crustal region in Luzon Philippine Department of Science and Technology and the Philip- which, in some places, coincides with major faults. In places pine Council for Industry and Energy Research and Development. where no physical boundary is observed, the interplay of various Discussions with members of the Rushurgent Working Group are magmatic, structural and sedimentation processes causes the acknowledged. boundary to be diffused and not readily discernible. The Central Cordillera Range corresponds to the volcanic/magmatic arc in Northern Luzon.

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Sc. degree from Dimalanta, C.B., 1996, Magnetic signatures of lithologic variation, fault the Ocean Research Institute, Uni- structures and hydrothermal mineralization: An example from the Baguio versity of Tokyo in September 2001. Mineral District, Luzon, Philippines: Journal of Southeast Asian Earth Her research interests include the Sciences, v. 14, pp. 1-10. use of gravity and seismic data to Dimalanta, C.B. and Yumul, G.P.Jr., 2003, Magmatic and amagmatic contri- determine the crustal thickness of butions to crustal growth of an island arc system: The Philippine exam- island arcs. ple: International Geology Review, 45, 922-935. Divis, A.F., 1980, The petrology and tectonics of recent volcanism in the Central Philippine Island, in Hayes, D.E., ed, The tectonic and geologic evolution of Southeast Asian Seas and Islands: Washington, D.C., Amer- G.P. Yumul, Jr. is a Professor at the ican Geophysical Union, pp. 127-144. National Institute of Geological Sci- Fitch, T.J., 1972, Plate convergence, transcurrent faults and internal defor- ences, University of the Philippines, mation adjacent to Southeast Asia and the Western Pacific: Journal of Diliman, Quezon City, Philippines Geophysical Research, v. 77, pp. 4432-4460. 1101. He is also currently the Execu- Godfrey, N.J. and Klemperer, S.L., 1998, Ophiolitic basement to a forearc tive Director of the Philippine Coun- basin and implications for continental growth: The Coast Range/Great cil for Industry and Energy Research Valley ophiolite, California: Tectonics, v. 17, pp. 558-570. and Development of the Philippine Hacker, B.R., Mosenfelder, S.C. and Gnos, E., 1996, Rapid emplacement of the Oman ophiolite: thermal and geochronologic constraints: Tectonics, Department of Science and Technol- v. 15, 1230-1243. ogy. He obtained his D.Sc. degree Japan International Cooperation Agency — Metal Mining Agency of Japan, from the Geological Institute, Univer- 1999, Report on Regional Survey for Mineral Resource in the Bicol sity of Tokyo in 1990. Most of his work Area: Tokyo. deals with the geochemistry and Mitchell, A.H.G. and Leach, T.M., 1991, Epithermal Gold in the Philippines: petrology of ophiolites and adakites Island arc metallogenesis, geothermal systems and geology: London, as well as the tectonics of the Philip- Academic Press Limited, 457pp. pines.

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